JTM45 Clone little problem!

voodoo.chile · Post by **voodoo.chile** » Mon Aug 22, 2005 7:04 am

Volleys to everybody, I have built a JTM 45 (before the Kit of George,but with Metropolous parts), that rings well, but a small problem has: from lit, also with cable of the disconnected tool, produces an incostant and variable rustle and hiss, which is accentuated increasing the TREBLE pot.
Help please!

I add a picture of BEAST!

Ricky Lee · Post by **Ricky Lee** » Mon Aug 22, 2005 12:44 pm

Looks like you have your preamp sockets turned 180 degrees from where they normally should be or you have the heater wires (red/black) in reverse. In any event, your wires connecting the preamp tubes are laying against the heater wires themselves, which is not good. Some of those wires in the preamp section don't live well together. Also try shortening some of those wires on the preamp sockets, as well as the green wires connecting the input jacks, (typically the shorter the better). I also noticed the bias resistor on the power tube socket v4 may be connected wrong, one lead for each socket terminal and then one lead to ground. I can't tell, but it appears you don't have a ground wire on the two Sprague caps going to the ground buss on the control pots. Typically hums, hisses and abnormally high noises are due to wire placement. Try moving some of the wires around to see if the noise changes. Here is a pic of the 45 I just finished and it's quiet as a mouse. Hope this helps.

http://img101.imageshack.us/img101/8994/jtm450ig.jpg

Guest · Post by **Guest** » Mon Aug 22, 2005 1:02 pm

Thanks for reply!
Precising that the ampli sound well, (I have sounded it for two mounts):

Bias Resistor:
I have linked pin 1 and 8 and then the resistor connected to ground. I think is correct, I read the bias values!

Tube socket:
Yes, the socket are rotate, is important the correct rotation?

The Sprague caps are grounded with the filter caps under the board.

The hum/hiss problem (incostant and variable), is present with and without the istrument cable insert.

I tried to move all wires, but I have only little changes and the hum remains.

I can solve the problem shortening all pots wires and imput wires?

Thanks

Ricky Lee · Post by **Ricky Lee** » Mon Aug 22, 2005 1:19 pm

Anonymous wrote:Thanks for reply!
Precising that the ampli sound well, (I have sounded it for two mounts):

Bias Resistor:
I have linked pin 1 and 8 and then the resistor connected to ground. I think is correct, I read the bias values!

Tube socket:
Yes, the socket are rotate, is important the correct rotation?

The Sprague caps are grounded with the filter caps under the board.

The hum/hiss problem (incostant and variable), is present with and without the istrument cable insert.

I tried to move all wires, but I have only little changes and the hum remains.

I can solve the problem shortening all pots wires and imput wires?

Thanks

Try removing the ground wire from the Sprague caps to the filter cap under the board and connecting it to the ground buss on the control pots. I would stiill connect the leads of the bias resistor from terminal to terminal and then one lead to ground. With the bias resistor leads connected terminal to terminal creates resistance between the two socket pins, setting up the bias as opposed to resistance between ground and both pins, the way it is now. Try and recheck the bias. Yes, the correct rotation of the preamp sockets is important. Notice the way they normally go puts the leads away from the red/black heater wires, not to mention staying uniform with the build itself.

voodoo.chile · Post by **voodoo.chile** » Mon Aug 22, 2005 1:23 pm

OK, I try this mods.

Thanks Ricky!

Ricky Lee · Post by **Ricky Lee** » Mon Aug 22, 2005 7:17 pm

Glad to help. If you need more assistance just ring. Just make sure you connect the bias resistors on V4 & V5 the way I explained. Typically the bias resistor lead nearest the rear of the chassis is the one that goes to the ground lug. Make your bias adjustments off the the tube with the highest ma reading.

voodoo.chile · Post by **voodoo.chile** » Wed Aug 24, 2005 5:46 am

I'm rewiring the amp, new heater and new preamp wiring, for solving the hum problem.

A little question: what the difference in the resistors on power tubes if is 1M or 470 ohm?

I have a 1M ohm.

Flames1950 · Post by **Flames1950** » Wed Aug 24, 2005 7:37 am

I hope 1M (meaning 1 megohm) is mis-typed. EL34's use a 1K, not a 1M -- KT66's will live OK on the 1K. 1 Megohm would drastically lower your screen voltage and probably adversly affect your amp's performance, at least as we Marshall guys like them to perform......

Flames1950 · Post by **Flames1950** » Wed Aug 24, 2005 7:39 am

I didn't remember you having the chassis picture up earlier. Looks pretty good, but your noise problems may benefit from shortening those green wires from the input jacks to the board. Especially the bright channel wires which are running a long ways over by the tone controls and may be a big part of your noise.

voodoo.chile · Post by **voodoo.chile** » Wed Aug 24, 2005 9:55 am

Sorry Flames!
The resistors are 1K ohm really!!

But the difference with the 470 ohm?

For the noise problem I'm rewiring all the board, but I think it not depend from the imputs wires, because the hum/hiss is present with and WITHOUT guitar cable in!

Flames1950 · Post by **Flames1950** » Wed Aug 24, 2005 3:33 pm

I don't think whether the guitar cable is in or not is the issue. I'm worried about those wires running close enough to the volumes and treble wiring to pick up some induced noise. They really need to be as short as possible.

Ricky Lee · Post by **Ricky Lee** » Wed Aug 24, 2005 6:33 pm

I looked at your chassis pic again and I see alot of unecessary long wire in several locations, not just the input wires but on the control pots and other places as well. As Flames said, the shorter the better and additionally would probably look a little neater if your concerned with cosmetics. Typically, I use just enough wire to go from point A to point Z with a lilltle extra length in case I haft to desolder and resolder a wire later down the road, especially those wires connecting the underside of the board. Also, the shortest distance is not always the best route with these amps, sometimes a little extra is needed to vector areas or other wires where one doesn't coexist well with the other.

Flames1950 · Post by **Flames1950** » Wed Aug 24, 2005 11:53 pm

I'd take the picture of Ricky's build and use it as a guide of how the wiring should route, as far as length and layout -- he did an immaculate job on his and it's a good example to work from. It should really help clean up your noise problems.

voodoo.chile · Post by **voodoo.chile** » Fri Aug 26, 2005 2:53 am

I have rewired ALL the board wires, the heater in the correct way, and the pots wires as shortened, but, when I take ON the ampli, is have the same irregular HUM / HISS!

The disturb is present ONLY when I pull high the TREBLE pot. When all the pots are 0 I don't have problem........

I think I have one or more caps broken on the line of TREBLE POTS............ IS POSSIBLE?

Help please!!!

I add a pic of new work when aivable.

Post by **VelvetGeorge** » Fri Aug 26, 2005 3:32 am

Does the sound go away when you remove the first preamp tube? How about the second one?

George